I recently tested a 5000mAh rated battery, but it was only 1000mAh. After the review I was asked if there was a small battery inside or something else, i.e. would I please open it up and check it.
Today it was nice weather and I did it (The nice weather is important, because this is an outdoor job).

I decided to use a tube cutter for it. It is easy to open a battery this way, but as I also found out, cut a little bit too deep and I short the battery. There is no risk of explosion, because the battery is already open, but it do get hot (See below).

The battery does look normal (no small battery inside).

Lets take a closer look at the top. It has vent holes below the white disc.
But more surprising is it that the button top is part of the battery, usual batteries are manufactured with flat top.

Here I have removed the top from the can. In the battery all the can is the minus pole and the top is the plus pole. The red ring works as electric isolation between them and as seal.
The metal foil on the back of the top is the pressure seal. I.e. if the pressure inside the battery gets very high, this seal will break (Hopefully) and let the pressure out through the vent holes, instead of the battery exploding.
Usual this pressure seal is a CID (Current Interrupt Device), that will also break the connection to the plus pole in addition to venting, but not in this battery.

Batteries does usual have a PTC (Positive Temperature Coefficient Resistor) in the top, to protect it against shorts.
I cannot see any in this battery.

A closer look at the body of the battery shows a normal battery construction. I could not get it out.

Here I have removed the bottom of the battery and a bit more of the actual battery construction can be seen.

Another 1000mAh battery

I have once before tested a 5000mAh rated battery with 1000mAh in it, lets compare construction.

Ouch, I did cut to deep and shorted the battery.
It looks like there is a second wrapper below the first one.

Bingo, anybody know what factory this is? (This is from Lishen and designed for power tools and a fairly old cell).
It says IMR, it might even be correct, because this battery does handle load pretty well.

After some cooling time I could work with it again. It looks like nearly the same construction (The blue washer was also present in the other battery, but I did not get a picture of it).
Only change is that the plus connection is welded to a intermediate plate and not directly to the pressure seal foil.

This cell is a flat top cell with a welded button top.

This look like the first battery.

Here I have unwinded a bit of the foil, it has four layers.

This is one electrode of the battery, the black stuff is the actual battery chemicals.
The foil is coated on both sides.

The other electrode of the battery, the black stuff is again the actual battery chemicals.
The foil is coated on both sides.

The separator, there are two of these. They must provide electric isolation between the two electrodes, but at the same time allow chemical processes through the foil.

Conclusion

Both batteries looks fairly normal. The safety is on the low side with no PTC and CID.
I do not know the reason for the low capacity, but a guesses could be that the foil and electrodes are thicker than usual and the actual battery chemicals are a very cheap type, maybe with a thinner coating than usual.
For the second battery it is because it is a high current cell.

My guess is that the first battery is made on a dedicated production line and only sold as flashlight/vapor batteries. I.e. it is neither a old battery nor second hand battery, but simply made as cheap as possible.

The second battery is probably old stock where a button top has been welded on and a new shiny wrapper added.